The application of ketimine derivatives in solid phase peptide synthesis

The N- [a-phenyl-(5-substituted-2-hydroxybenzylidene)] -amino acids react smoothly with amino acyl polymers in the presence of N,N'-dicyclohexylcarbodiimide (DCC) to afford the ketimine peptide resin esters in quantitative yield. Several N-protected dipeptide resin esters were synthesized and characterized as their methyl ester derivatives. The ketimine protecting group was found to be remarkably stable to hydrolysis when incorporated into a Merrifield resin, in fact no suitable method for its removal from peptide resins could be developed. This unusual stability of the diarylmethylene protecting group when incorporated into a Merrifield type styrene divinylbenzene polymer support suggested its suitability for the side chain protection of ornithine and lysine in solid phase peptide synthesis.